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Durability is the most important criterion used to define the quality of a tool steel. Cryogenictreatment and tempering of metals has been ac-

knowledge for almost thirty years as aneffective method for increasing durability, or "wear life" and decreasing residual stress in toolsteels. Deep cryogenics (below-300°F) is creating many new applications in science. Hightemperature superconductors, the super-conducting super collider, cryo-biology, magneto-hydrodynamic drive systems for ships, and low temperature physics have all developedrecently. The deep cryogenic treatment and tempering process for metals is economical. It isa one time permanent treatment, affecting the entire part, not just the surface. The treatmentmay be applied to new or used tools, sharp or dull, and reshaping will not destroy theimparted properties. Benefits achieved from subjecting tools to this treatment include:increases in tensile strength, toughness, and stability through the release of internal stresses.

Cryogenic Treatment for Improved Properties

A research metallurgist at the National Bureau of Standards in Boulder Colorado, states,"When carbon precipitates form, the internal stress in the martensite is reduced, whichminimizes the susceptibility to micro cracking. The

wide distribution of very hard, finecarbides from deep cryogenic treatment also increases wear resistance." The study concludes:"...fine carbon carbides and resultant tight lattice structures are precipitated from cryogenictreatment. These particles are responsible for the exceptional wear characteristics imparted bythe process, due to a denser molecular structure and resulting larger surface area of contact,reducing friction, heat and wear." There have been skeptics of the cryogenic process for sometime, because it imparts no apparent visible changes to the metal. Since proper heat treatingcan transform 85% of the retained austenite to martensite and the deep cryogenic processonly transforms an additional 8 to 15%, the deep cryogenic treatment hasbeen considered aninefficient process.

While these percentages are correct, the conclusion drawn from them is inaccurate. Inaddition to the trans-

formation to martensite, the subjected metals also develop a moreuniform, refined microstructure with greater density. Although known to exist, this type ofmicrostructure was only recently quantified scientifically. Particles known as "binders" arecoupled with the precipitation of the additional micro fine carbide "fillers". The fillers take upthe remaining space in the micro-voids, resulting in a much denser, coherent structure of thetool steel. These particles are identified and counted in the above study cited, using ascanning electron microscope with field particle quanti-

fiction (an automatic particlecounter). It is now believed that these particles are largely responsible for the great gains inwear resistivity. The permanent irreversible molecular change created is uniform throughoutthe tool, unlike coatings, and will last the life of the tool, regardless of any subsequentfinishing operations or regrinds.

Deep Cryogenic Treatment Potential

The cryogenic cycle is an extension of standard heat-treatment, and creates many outstandingincreases in durability. Some examples are as follows. A major aircraft manufacturer testingdeep cryogenic treatment found that with only six different tools treated, the savings in toolpurchases could exceed $5 million. An Arizona State study conducted by Laurel Hunt, useddeep treated C-2 debarring tools on INCONEL

alloy 718, achieving a 400% improvementbased on weight, after five cats of .003 in. (.007 cm) on this alloy. This deep cryogenictreatment of an 8% cobalt end mill has made dramatic improvements in two important ways.

The number of milling cats was increased from three before deep cryogenic processing, to 78cats after processing (26 times the wear life). Resharpening the end mills after deep cryogenictreatment required only 1/3 the amount of stock removal to restore the tool geometry.Rockwell, a major

aircraft manufacturer, using C-2 carbide inserts to mill epoxy graphite,doubles their output after deep cryogenic treatment of the inserts. In a second test, a 400%improvement was achieved upon milling 4340 stainless steel with cryogenic treated tool.Other applications include: Leading national stock car drivers who previously raced only 4-8races between equipment teardowns, drove in 40+ races before teardown after cryogenicallytreating block, crank, cam, pistons and heads.